An Opinion Poll is asking your "opinion" via a vote count. It really isn't necessary to post a written opinion unless of course, you want folks to know or have something of substance to speak about.
To give an example, there are two polls running on the homepage. The FW
and OT pics of the month. At the end of each pic, there is a suggestion that if you want to comment on them to do so in the appropriate forum.
I've been a member for several months and I have yet to see this done.